The present invention is concerned with the field of leak detectors for pipes. The present invention is more specifically concerned with an apparatus for determining the position of a leak in an underground pipe for fluid or gas. The apparatus can also be used to detect the position of the pipe itself.
A leak in a pipe generates an acoustic signal which propagates along the walls of the pipe and the fluid inside the pipe. Thus, a leak in a pipe can be detected by listening for the acoustic signal associated with a leak.
Initially, a leak is determined to an accuracy of say, about, 150 meters using a leak detection apparatus such as an acoustic noise logger. An acoustic noise logger may constantly or periodically record acoustic signals from a length of pipe. Other leak detection apparatus could be used. For example, some leak detectors measure changes of the average pressure within the pipe, variations in the input and output volumes of fluid supplied to the pipe, temperature variations etc to detect the leak. After the position of the leak has been roughly determined, it is necessary to precisely locate the leak so that a hole can be excavated and repair accomplished.
There are known methods for accurately determining the position of a leak, for example, the signal from the leak can be measured by two fixed acoustic sensors, located on either side of the leak. The position of the leak can then be determined by comparing the time of arrival of the leak signal at the two fixed sensors to determine the position of the leak. This correlation method suffers from the problem that, for the method to be effective, the velocity of sound in the pipe needs to be accurately known. The speed of sound will vary depending on the pipe material and diameter. It is a complex calculation, as often, the pipe material and diameter will change over a single stretch of pipe. Also, many other factors can affect the velocity calculations such as temperature which nay fluctuate along the length of the pipe.
Often a correlator is used in conjunction with a ground microphones The ground microphone is used to confirm the results of the correlation technique.
To use the ground microphone, the operator stands over the predicted position of the leak and the microphone is moved over the ground noting changes in the leak noise and intensity. The distance from the leak to the ground microphone will be shortest when the ground microphone is directly above the leak. Hence, there will be a maximum in the measured noise intensity directly above the leak. Determining the position of a leak using a ground microphone is a highly skilled task, as the leak noise can easily be affected by extraneous noises such as wind, traffic etc. Hence, results from using a ground microphone are extremely variable and operator dependent.
The present invention seeks to address the above problems and, in a first aspect, provides an apparatus for determining the position of a signal from a pipe, the apparatus comprising a first sensor capable of being coupled to said pipe for detecting a signal from said pipe, a second sensor being moveable with respect to said pipe for detecting a signal from said pipe and determining means for determining a change in the signals detected by the first and second sensors due to the difference in position of the first and second sensors with respect to the pipe.
The present invention does not require an accurate estimate of the speed of sound through a pipe or through the ground. In use, the first sensor is coupled to the fluid pipe and is used to detect sound either carried along the walls of the pipe or along fluid in the pipe. The fluid in the pipe can be a liquid or a gas. If there is a leak in the pipe, this leak will have a characteristic sound which can be recognised by those skilled in the art. The first or fixed sensor is used to detect this signal. A second or mobile ground sensor is then moved by an operator over the ground above the suspected vicinity of the leak.
This mobile sensor will also detect the acoustic signal due to the leak. Both the mobile sensor and the fixed sensor are used together and simultaneously to detect the leak.
The change in the signals will be due to the different times which the signal takes to propagate from the pipe to the first and second sensors. This change in the detected time of arrival of the signal may be detected as a temporal shift between the two signals if detected in the time domain. However, it will also be appreciated that the signals could be detected using the frequency domain.
The determining means generally calculates the time delay between the signal arriving at the fixed sensor and the mobile sensor, the time delay may be measured in the time or frequency domain. The actual time which the signal takes to reach the fixed sensor will be constant throughout the measurement. However, the time which the signal takes to reach the mobile sensor will depend on the distance between the mobile sensor and the leak. Therefore, the calculated time delay will depend on the position of the leak relative to the mobile sensor.
The pipes will, typically be at most 10 meters underground. The sensor will usually be coupled to the pipe at possibly, 5 kilometer intervals. As a result, the mobile sensor will almost always be closer to the leak than the fixed sensor. Thus, the mobile sensor will receive the signal before the signal is received by the fixed sensor. Therefore, generally, when the time difference between the detected signal at the mobile sensor and the fixed sensor will be a maximum, the mobile sensor is directly above the leak. The determining means may comprise means for indicating a maximum in the time difference between the signals detected by the first and second sensors.
The apparatus of the present invention can thus determine the position of a leak without the need to accurately determine the velocity of sound in the pipe. Also, as the position of the leak is usually signalled by a maximum in a measured time delay, then little operator skill is required to detect the exact position of the leak.
Preferably, the determining means which measures the time delay between the detected signals from the fixed and mobile sensors will be located with the mobile sensor. The mobile sensor and determining means may be provided in a single unit which the operator can move over the ground and which will indicate when the time delay is a maximum. Alternatively, the mobile sensor may be housed in a hand held housing and the comparing means may be in a separate unit which may be located, for example, around the operator""s neck or may be located in the operator""s car, van etc. The determining means could be connected to the mobile sensor by a hard link such as a cable or a wireless link. Thus, the determining means could be remote from both of the sensors and both sensors could comprise a transmitter or the like.
In order for the fixed sensor to be able to contact the comparing means which will be remote from the fixed sensor, the fixed sensor comprises transmitting means for transmitting data, corresponding to the signal, to the comparing means. These transmitting means are preferably radio transmission means. However, it will be appreciated that they could also be a physical link between the fixed sensors and determining means, such as a wire. The transmission means may be configured to transmit analogue or digital signals from either or both of the first and/or second sensors.
The sensor preferably comprises means which allow the sensor to be coupled to the pipe. These means may be in the form of a member which allows the sensor to be coupled to the pipe. Alternatively, the means may be integral with the sensor itself, for example, the sensor, or its housing may be shaped to be inserted into an access point of the pipe or shaped to allow external coupling to the pipe.
It should be noted that the fixed sensor only needs to be coupled to the pipe during leak detection. The fixed sensor is preferably not permanently coupled to the pipe. Typically, the fixed sensor will be coupled to the pipe at some access point to the pipe such as a valve, stop tap etc. The sensor may be magnetically coupled to the pipe.
Alternatively, the sensor may be a hydrophone or the like which is inserted into the fluid of the pipe via a hydrant or the like.
The mobile sensor is preferably capable of detecting the signal when the sensor is in motion as well as when the sensor is stationary.
The mobile sensor will preferably be a piezo-electric sensor. More preferably, a ceramic construction which is enclosed to minimise the impact of external traffic and ambient noise. To reduce traffic and ambient noise, the sensor will preferably comprise filters configured to filter out traffic and ambient noise.
The sensor is configured to be placed upon the ground surface. Therefore, the design needs to be adapted to ensure good acoustic contact. Preferably, the sensor will be located in a dome-shaped structure which sits with its flat or underside surface on the ground or is positioned with this surface substantially parallel to the ground.
In order to obtain a clear signal, it is preferable if the signal detected by at least one of the fire and second sensors is filtered. More preferably, the signal detected by both of the first and second sensors will be filtered. The filtering of the first and second signals may be controlled by the determining means such that the determining means receives optimised signals.
The present invention can also be used for the detection of an actual pipe itself as opposed to a leak. There is a problem in that in many pipe networks, the exact position of a pipe is unknown. A noise can be made in the pipe by, for example, hitting the pipe. The noise propagates along the length of the pipe. This can be detected in the same way as the leak. The actual position on the pipe can be determined by the mobile sensor as the maximum time delay between the signals detected by the mobile sensor and the fixed sensor will be measured when the sensor is directly above the pipe.
In a second aspect, the present invention provides a method of determining the position of a signal originating from a pipe, the method comprising the steps of:
(a) coupling a first sensor to a pipe, wherein the sensor can detect a signal from said pipe;
(b) moving a second sensor over the ground above said pipe and detecting a signal from said pipe;
(c) comparing the signals detected by the first sensor and the second sensor in order to determine a difference between the detected signals due to the difference in the position of the first and second sensors with respect to the pipe.